Electronic charge transfer in stage-2 fluorine-intercalated graphite compounds

The optical reflectivity in the visible and the near infrared of fluorine-intercalated highly oriented pyrolytic graphite compounds was studied as a function of fluorine concentration between ${\mathrm{C}}_9$F and ${\mathrm{C}}_{3.4}$F. Between ${\mathrm{C}}_8$F and ${\mathrm{C}}_4$F we studied the samples which exhibited a stage-2 structure. The experimental results were analyzed in terms of the Blinowski-Rigaux two-dimensional (2D) model, which yields directly the plasma frequency and the scattering time of the free charge carriers. It is found that, for fluorine concentrations below that of ${\mathrm{C}}_6$F, the plasma frequency increases with increasing concentration and has a maximum at ${\mathrm{C}}_6$F. At higher fluorine concentrations, the plasma frequency decreases rather sharply. This anomalous behavior is explained in terms of a two-acceptor-state model. Below the fluorine concentration of ${\mathrm{C}}_6$F, there is only one acceptor state with an energy $E_1$=$E_F$-1.034 eV. A second acceptor state forms when the concentration of fluorine increases further. Each additional fluorine transfers two other fluorines from the first acceptor state to the second, the energy of which is $E_F$-0.84 eV. The behavior at still higher fluorine concentrations is different again and indicates the formation of covalent bonds. Our analysis indicates that in the limit of low fluorine concentrations the charge transfer is one electron per six fluorine atoms. It drops as the concentration increases and is (1/8 per fluorine at ${\mathrm{C}}_6$F and (1/16 at ${\mathrm{C}}_4$F. These results agree very well with the two-acceptor-state model.